As output apparatuses of computers and workstations, electrophotography type of image-forming apparatuses are known which form images on a recording medium by use of a developing agent (toner). With the electrophotographic image-forming apparatus, for example, an electrostatic latent image is formed by projecting a light beam (e.g., laser beam) carrying image information onto an image holding member such as a photosensitive drum; a toner is supplied onto the electrostatic latent image by a developing roller to form a developed image; the developed image is transferred by a transferring roller onto a recording medium to form a transferred image (developed image); and the recording medium having received the transferred image is delivered to a fixing assembly to fix the transferred image on the recording medium.
The fixation assembly generally comprises a fixing roller having a built-in heater and a pressing roller in pressure contact with the fixing roller. For fixation of the transferred image on the recording medium, the fixing roller and a pressing roller pinch and deliver the recording medium to heat the recording medium to a prescribed fixation temperature and simultaneously press it. The transferred image is fixed by heat and pressure. The recording medium holding the fixed image is taken out of the fixation assembly by discharge rollers.
The fixation assembly is explained by reference to FIG. 29.
FIG. 29 is a diagram illustrating schematically a conventional fixation assembly.
The fixation assembly 100 visualizes permanently the toner (image) 102 on a recording medium 104. The recording medium 104 delivered in the arrow-A direction by a delivery unit (not shown in the drawing) is guided by fixation inlet guide 106 and is introduced to a nip 108 between a fixing roller 120 and a pressing roller 130.
The fixing roller 120 heats and melts the toner. A thermistor 140 is in contact with the peripheral face (surface) of the fixation roller 120 to monitor the temperature of the peripheral face of the fixation roller 120. The fixation roller 120 has a built-in heat source (heater) like a halogen heater 122. The halogen heater 122 is controlled at a prescribed fixation temperature by a controller (not shown in the drawing) by reference to the peripheral surface temperature measured by the thermistor 140 to keep the outside peripheral temperature of the fixing roller 120 at a prescribed level.
Generally the fixation roller 120 is constituted for example, of a core metal 124 made of an iron or aluminum pipe-shaped member coated with a releasable fluoro-resin layer 126. The fixing roller 120 is rotated in the arrow-B direction by a driving source (not shown in the drawing).
The pressing roller 130 presses the recording medium 104 at a prescribed pressure against the fixation roller 120. The pressing roller 130 is constituted, for example, of a core metal 132 coated with an elastic material layer 134 such a layer of silicone rubber and fluro-rubber in a prescribed thickness the peripheral face thereof. The pressing roller 130 is pressed against the fixation roller 120 at a prescribed pressure and is rotated in the arrow C direction to apply pressure for fixation of the toner 102 onto the recording medium 104.
When the recording medium 104 has entered the nip 108, the toner 102 on the recording medium 104 is fused at the aforementioned fixation temperature, and the fused toner 102 is pressed against the recording medium 104 with the aforementioned load and is fixed on the recording medium 104. The recording medium 104 carrying the toner 102 fixed thereon is released from the fixing roller 120 and the pressuring roller 130 by a releasing nail 142 to reach a sheet-discharging roller (not shown in the drawing) and is discharged out of the apparatus.
A fixing roller 120, which is manufactured by working of an aluminum pipe, for example, is made to have a smaller outside diameter by about 0.07-0.2 mm at the lengthwise middle portion of the fixing roller 120 than the outside diameter at the both end portions of the fixation roller 120 (so-called inverse crown shape). The larger diameter at the both lengthwise ends of the fixing roller 120 than the lengthwise middle portion thereof makes larger the peripheral speed of the fixing roller 120 at the both lengthwise end portions to deliver the recording medium 104 by pulling it outward (so-called inverse crown effect). Consequently, the recording medium 104 can be delivered without wrinkling.
The warming-up time of the fixing roller 120 is preferably shorter in view of energy saving. A certain image-forming apparatus is capable of finishing and discharging a first copy sheet within 30 seconds (warming-up time) after turning on the main switch from a complete cool state of the image-forming apparatus main body. The warming-up time is becoming shorter year by year.
In a standby state in which the main switch of the image-forming apparatus is kept turned on, the power consumption for keeping the fixing assembly in a warm state is desirably less. For decreasing the power consumption during the standby state, preferably the heater of the fixing assembly is turned off completely. In the case where the heater of the fixation assembly is turned off completely, for heating the fixing roller immediately up to the prescribed temperature, the wall thickness of the fixing roller is made thinner to decrease the heat capacity thereof. Therefore, the fixation roller is usually made of aluminum alloy having a high thermal conductivity.
In order to shorten the warming-up time, recently the wall thickness of the fixing roller 120 is decreased to as thin as 0.8 mm. With a smaller wall thickness of the fixing roller 120, the fixing roller may be deformed in fixation of the image by heat and pressure by holding the recording medium 104 between the fixing roller 120 and the pressing roller 130 (at the nip 108). With the fixing roller 120 having a smaller cylinder diameter at the lengthwise middle portion than at the lengthwise ends, the middle portion is liable to be deformed to lower the fixation performance at this lengthwise middle portion.
To solve the above problems, insertion of a spring coil into the interior of the fixing roller 120 is disclosed to reinforce the fixing roller 120 (Japanese Patent Application Laid-Open No. 10-116675). In another technique, a rib is formed in a spiral state on the inside peripheral wall of the fixing roller 120 to reinforce the fixing roller 120 (Japanese Patent Application Laid-Open No. 2000-29342)
However, the fixing roller having a spiral spring or spiral rib on the inside peripheral face has locally a higher strength at the portion opposite to the inside spiral portion. As the results, the outside peripheral portion corresponding to the inside spiral member will give a higher nip pressure than other portions to decrease the inverse crown effect. Moreover since the portion of the higher nip pressure is spiral, the recording medium is allowed to deviate toward one lengthwise end side of the fixing roller. Thereby the recording medium may come to be fed obliquely or may be wrinkled to cause failure in delivery of the recording medium and to make instable the fixation of the transferred image on the recording medium.